1. Technical Field
The embodiments herein generally relate to nonaqueous electrolytes that support the operation of electrochemical devices with high cell voltages, and more particularly, to the solvents and additives that form the nonaqueous electrolytes and can stably support the cell chemistry of the electrochemical devices with high cell voltages.
2. Description of the Related Art
The electrochemical devices that output high cell voltages utilize nonaqueous and aprotic solvents to dissolve the conducting salts, because these solvents are able to afford the stability against the oxidative or reductive reactions incurred by electrode surfaces of extreme potentials. Because the electrolyte components are almost never thermodynamically stable on the strongly reductive surfaces of anode or strongly oxidative surfaces of cathode, the electrochemical stability is rather attained through the passivation of the electrode surfaces. The above passivation is realized by the initial decompositions of solvents in trace amount and the concomitant deposition of these decomposition products which deactivate the catalytic sites of the electrode surfaces. Generally, all electrochemical devices that produce cell voltages higher than 3.0 V, and particularly in lithium-based battery chemistries, certain solvents were developed in the conventional solutions so that their decomposition products on anode and cathode surfaces are able to form dense and protective passivation layers. These solvents include ethylene carbonate (EC), vinylene carbonate (VC), and other polar and aprotic solvents and/or additives, and have become indispensable components in commercial Li ion batteries. In other words, the conventional Li ion batteries operate at high voltages (3-5 V), which were made possible by the passivation film formed on the surfaces of the anode and/or cathode. While providing protection, the film also presents resistance to the kinetics of the cell chemistry, rendering poor power density as well as poor low temperature performances.
However, the passivation layers formed by the above-described solvents and/or additives in conventional electrolytes also constitute the most resistive component in the electrochemical cells, which not only compromises the cell performances at low temperatures but also impose the kinetic restrictions on the power density of the devices at room temperature.
Furthermore, as the battery chemistries of higher energy density are being pursued, cathode materials of higher potentials become focus of research efforts, and the passivation layer formed by these solvents or additives of the conventional solutions can no longer ensure the stable operation of the cell chemistry.